The use of seatbelt pretensioners for pretensioning a seatbelt in the event of a collision is common in the art. Pretensioners are used, generally, to pretension a seatbelt during a collision, which will remove the slack and cause the seatbelt to become tighter on the occupant wearing the seatbelt. Tightening the seatbelt in the event of a collision can limit the amount that the occupant will travel forward during a crash (excursion) and can reduce peak loads acting on the occupant during a crash sequence. Pretensioners are generally activated by sensors in the vehicle that determine that a collision has occurred (or is imminent).
One type of pretensioners is known as a linear pretensioner, and typically involves the use of a piston that travels linearly propelled by gas produced by a pyrotechnic gas generator. A cable or other element is coupled to the piston and a component of the seat belt system thereby linearly pulling on the belt in a single direction. The piston can be attached to an end of the seatbelt or a seatbelt anchorage and can be coupled to the lap belt or shoulder belt section of a three-point belt system, or both. Presently available linear pretensioner devices are limited in the amount that the seatbelt can be pulled by the stroke of the piston. Space constraints within the vehicle can limit the size and length of the piston, thereby limiting the length that the belt can be pulled. Along with cost concerns, packaging constraints are always present in a motor vehicle component design and implementation. Accordingly, compact and cost-effective designs are highly desirable for automotive manufacturers and their suppliers.
Another type of seat belt pretensioner is in the form or a rotary pretensioner. One type is a so-called pyrotechnic rotopretensioner. A pyrotechnic rotopretensioner generally includes a rotatable spool with a sprocket or pinion having radially extending teeth. A series of masses such as ball masses are arranged in a string and are guided through a tube connected to a gas generator. In the event of a collision, the gas generator is activated to propel the ball masses along a guide path toward the spool sprocket. The ball masses interact with the spool sprocket in a generally tangential manner, making contact with the teeth and causing the spool to rotate about a spindle. The spindle is connected to the seatbelt webbing, causing the seatbelt to become wound, thereby shortening the length of the belt and pretensioning the belt. Although pyrotechnic rotopretensioners perform well, there is a continuing desire to reduce packaging space requirements.